Java Control statements

Java Control statements control the order of execution in a java program, based on data values and conditional logic. There are three main categories of control flow statements;
· Selection statements: if, if-else and switch.
· Loop statements: while, do-while and for.
· Transfer statements: break, continue, return, try-catch-finally and assert.
We use control statements when we want to change the default sequential order of execution

Selection Statements

If Statement

The if statement executes a block of code only if the specified expression is true. If the value is false, then the if block is skipped and execution continues with the rest of the program.

The simple if statement has the following syntax:

if (<conditional expression>)
<statement action>

public class IfStatementDemo {

      public static void main(String[] args) {
          int a = 10, b = 20;
          if (a > b)
             System.out.println("a > b");
          if (a < b)
             System.out.println("b > a");
     }
}

Output

b > a

If-else Statement

The if/else statement is an extension of the if statement. If the statements in the if statement fails, the statements in the else block are executed.

The if-else statement has the following syntax:
if (<conditional expression>)
<statement action>
else
<statement action>

public class IfElseStatementDemo {

     public static void main(String[] args) {
          int a = 10, b = 20;
          if (a > b) {
               System.out.println("a > b");
          } else {
               System.out.println("b > a");
      }
   }
}

Output
b > a

Switch Case Statement 
The switch statement begins with a keyword, followed by an expression that equates to a no long integral value. Following the controlling expression is a code block that contains zero or more labeled cases. Each label must equate to an integer constant and each must be unique. When the switch statement executes, it compares the value of the controlling expression to the values of each case label. The program will select the value of the case label that equals the value of the controlling expression and branch down that path to the end of the code block. If none of the case label values match, then none of the codes within the switch statement code block will be executed. Java includes a default label to use in cases where there are no matches. 

switch (<non-long integral expression>) {
case label₁: <statement₁>
case label₂: <statement₂>
…
case label_n: <statement_n>
default: <statement>
} // end switch
 
When executing a switch statement, the program falls through to the next case. Therefore, if you want to exit in the middle of the switch statement code block, you must insert a break statement, which causes the program to continue executing after the current code block.

Below is a java example that demonstrates conditional execution based on nested if else statement condition to find the greatest of 3 numbers.

public class SwitchCaseStatementDemo {

    public static void main(String[] args) {
         int a = 10, b = 20, c = 30;
         int status = -1;
         if (a > b && a > c) {
              status = 1;
         } else if (b > c) {
              status = 2;
         } else {
              status = 3;
     }
      switch (status) {
           case 1:
                 System.out.println("a is the greatest");
                 break;
           case 2:
                 System.out.println("b is the greatest");
                 break;
           case 3:
                System.out.println("c is the greatest");
                break;
           default:
                System.out.println("Cannot be determined");
      }
   }
}

Output
c is the greatest


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